The present invention relates in general to testing electric motor components and, more particularly, to methods for determining multiple winding or coil resistances and multiple coil connection or weld resistances within armatures used in electric motors.
Many electric motors and generators utilize armatures having a plurality of windings or coils which are connected in series to one another. The ends of the coils are connected to commutator bars such that each connection between two coils is connected to a commutator bar. The commutator bars are brought in and out of contact with brush type contacts to intermittently connect the coils to an electric power source for a motor or the electrical load for a generator.
The connections between the coils and the commutator bars are commonly formed by welds or solder joints. Since reliable welds are very low in resistance compared to the resistance of the coils of an armature, testing armatures through the commutator bars is challenging. For example, in a typical appliance motor, the resistance across a pair of commutator bars is in the order of 200 milliohms .+-..apprxeq.5%, .apprxeq.10 milliohms, whereas the maximum permissible contact resistance is approximately 0.1% of that amount, or about 0.2 milliohms. With such great differences between the resistance of a coil and the resistances of the connections or welds which connect it to its commutator bars, the direct measurement of the resistance between two commutator bars can not be used to detect a defective connection.
A number of testing arrangements are available to overcome these difficulties and permit accurate testing of armatures through the commutator bars. One example, U.S. Pat. No. 5,140,276, discloses an arrangement for testing the resistances of armature commutator connections and the resistances of individual coils of an armature. Two independent constant current sources having identical output currents are connected across opposite pairs of armature windings through their respective commutator bars.
The current sources are connected in opposite polarity to minimize circulating current in other windings of the armature. The voltages across windings adjacent to the two windings are combined and are a function of the combination of the resistances of two of the connections of the windings and the commutator bars. The voltages across each constant current source is representative of the resistances of the windings to which the sources are connected. Unfortunately, only combinations of two connection resistances can be determined with this arrangement and only one such combination for each measurement. The arrangement is also limited to the determination of two winding resistances at one time.
Another example, U.S. Pat. No. 5,307,019, discloses an arrangement wherein first electric stimuli are applied to pairs of commutator bars to measure the response at other pairs of commutator bars and thereby determine the resistance ratios between the coils of the other pairs of commutator bars. Second stimuli are then applied and response voltages measured between some other pairs of commutator bars to determine weld resistances based on the previously determined coil resistance ratios. Unfortunately, this repetitive two step arrangement requires repeated connections to an armature or relatively complicated control of a multiple connection jig or fixture for testing the armature.
There is an ongoing need for improved test arrangements for commutated armatures which serve not only to expand the art but also to simplify the testing of armature windings or coils and associated connections of the coils to commutator bars. Preferably such arrangements require less circuitry while testing armatures more quickly than prior art arrangements.